Artificial reconstruction of speech



Nov. 11, 1958 E. E. DAVID, JR., ETAL 2,860,187

ARTIFICIAL REcoNsTRUcTIoN -oF SPEECH 5 Sheets-Sheet 1A Filed Dec. 8,1955 E. E. DAV/QUE; S. Mc DOA/,4m

A TTORNEV Nov. l1, 1958 E. E. DAVID, JR., ETAL ARTIFICIAL RECONSTRUCTIONOF SPEECH Filed Deo. 8, 1955 5 Sheets-Sheet 2 l/s 4/ Y 2/3 4E 27 2711 E7l f TA PPEo TA PPE@ TAPPED TAPPEO OELAY L/NE DELAY L/NE DELAY L/NE DELAYL/NE CONTACTS OE CONTACTS of' cONTAGTS OP CONTACTS OE RELAY TREE RELAYTREE RELAY TREE RELAY TREE L l .J r, //r* cO/LS OF RELAY TREES ANO SELEc TOR SW/TcH 2\6 /23 NAR/ER ANALOG PULSE LP E. To O/G/TAL TT 50 GEN.CONVERTER MARKER l L PULSES FROM TP GEN. 2G +1 2*- PHANTASTRON V] |'1NONOSTAELE D Y N. P/TcH ELA GE VOLTAGE \5/ RESET f 55 PROM P/L TER 235T? S/4b 54@ 52 PULSE GEN j?) SGALE SCALE 1# OF OF /00 KC- TWO TWO \SS fS627 /Nl/E/VTORSE 5 DAV/0. JR

H. S. MCOONALO Nov. 11, 1958 E. E. DAVID, JR., ET AL 2,860,187ARTIFICIAL REcoNsIRucTIoN oF SPEECH 5. E. DAV/0, 'J/a /NVENTORSH s. Mc00A/ALD 554% CMMI',

A TTOR/VEV i Nov. 11, 1958 E; E. DAVID, JR., ET AL 2,860,187

ARTIFICIAL REcoNsTRucTIoN oF SPEECH Filed Deo. 8, 1955 5 sheets-sheet 40 OUTPUT 5 E DAV/0, JR.

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A NORA/5y E. E. DAVID, JR., ETAL ARTIFICIAL RECONSTRUCTION OF SPEECHNov. 11, 1958 5 Sheets-Sheet 5 Filed DSG. 8, 1955 .5. E. DAV/0J@/NVENTORS/f. 5. McDo/VAL@ ATTORNEY` United States Patent O ARTIFICIALRECONSTRUCTION OF SPEECH Edward E. David, Jr., Berkeley Heights, andHenry S. McDonald, Summit, N. J., assignors to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication December 8, 1955, Serial No. 551,940

14 Claims. (Cl. 17915.55)

This invention relates to economy of frequency bandwidth in telephony,and particularly to the artificial reconstruction of speech from narrowband transmitted signals. The principal object of the invention is to`improve the quality of such artificial speech.

It has long been known that a speech wave is almost periodic incharacter, the difference between the wave of each period and the waveof the following period being small compared with the variations of thewave within any period. Many proposals have undertaken to turn thissituation to account in the reduction of the band of frequenciesrequired for the transmission of the information content of the speechwave. According to one such proposal, described by H. W. Dudley in hisPatent 2,115,803, May 3, 1938, an electrical speech wave originating,for example, in a microphone, is chopped in synchronism and in phasewith its fundamental period and certain full periods, for example everyother one, are discarded. The retained periods are transmitted to areceiver station, preferably after stretching each one in the timedimension to fill the gap left in the wave train by elimination of thediscarded period. Such stretching carries with it a correspondingreduction in the frequency band required for transmission. At thereceiver station the received wave is resto-red to its originaldimensions in time and frequency, thus leaving blank intervalscorresponding to the eliminated periods. The teaching of the Dudleypatent is to fill each of `these gaps or blank intervals with one ormore repetitions of the last received period. As a result thereconstructed wave comprises a sequence of identical periods, followedby another sequence of mutually identical periods, and so on. The pricepaid for the bandwidth economy in transmission is therefore an increasein the abruptness with which the waveform of each such group of periodschanges to that of the next such group. Thus, for example, when two outof each group of three successive periods of a changing speech wave areeliminated at the transmitter, i. e., with a chopping factor N=3, thetransmission band can in principle be reduced by a factor of three; butthe abruptness of transition, at the receiver, from the last of eachgroup of three like periods to the first of the following group of threelike periods, is likewise increased by a factor of three.

This abruptness of transition naturally makes Ifor noise andunnaturalness in the reconstructed speech. It is a specific object ofthe present invention to reduce the noise and distortion which arise ina chopped speech transmission system for these reasons. This object isattained, in accordance with the invention, by inserting in each blankinterval of the received wave, not merely a repetition of the lastreceived period, but a modication of it. One such modification is aweighted average of the two waves which respectively precede and followthe blank interval. Thus, if the blank interval is only a single periodin length, (i. e., if the chopping factor N has the value 2) thepreceding and following waves are weighted alike `and the modified waveis obtained by 2,860,187 Patented Nov. 11, 1958 equal mixture, while thethird is filled with a mixture` of one part of the preceding wave andthree parts of the following wave. The apparatus `thus averages thewaveforms of earlier and later transmitted waves, and from such averagesconstructs artificial waves and inserts them in the blank intervals.

One of the principal differences between each period of a speech waveand the next is a difference in ampli-` tude. Indeed, some experimentalevidence indicates that this is the most important single difference.Accordingly,

the invention also provides a simplified system. in which' amplitudedifferences are taken account of in the intercalation of artificial waveperiods between successive transmitted wave periods, while disregardingother differences. In this simplified system an auxiliary amplitudecontrol signal is derived at the transmitter station and is transmitted,along with the chopped speech waves, to the receiver station where itcontrols the amplitudes of repetitions of each transmitted speech Wavefor insertion in the blank interval which follows it.

To provide that the auxiliary gain control signals shall affect only therepetitions of the received Wave and not the received period on itsfirst occurrence requires switch p ing apparatus of some complexity,operating in syntion terminals of a switchmorgate 5 to whosepontrol.`

chronism with the fundamental pitch frequency or a subharmonic thereof.In accordance with a further feature of the invention thesecomplications are avoided by use of compandor apparatus including anamplitude compressor at the transmitter station and a correspondingamplitude expandor at the receiver station. The amplitude controlsignal, which modifies the amplitudes of the repetitions of the receivedwave at the receiver station, may conveniently be derived from thecompressor at the transmitter station. With this arrangement theamplitude control signal may be applied to the final output signal atthe receiver station including each originally received period as wellas its various repetitions.

The invention will be fully apprehended from the following detaileddescription of preferred embodiments thereof taken in connection withthe appended drawings in which:

Fig. l is a block schematic diagram showing a transmission systemembodying the invention;

Fig. 2 is a Vschematic circuit diagram showing a weighting network foruse in the apparatus of Fig. 1;

Figs. 3 and 4 are block schematic diagrams showing electronicalternatives to the interpolator of Fig. l;

Fig. 5 is a block schematic diagram showing an electronic version of thetime scale stretcher of Fig. 1;

Fig. 6 is a group of waveform diagrams of assistance in explaining theoperation of Fig. 5;

Fig. 7 is a block schematic diagram showing an elec-` passed intoatransmission path 2 and two control paths 3, d. The transmission path 2includes the two conducd terminal a control signal derived in the firstcontrol path 3 is applied.

The first control path 3 comprises a period marker pulse generator whichmay advantageously be of the type which forms the subject matter of E.Peterson Patent 2,593,694, issued April 22, 1952, and which is furtherdescribed by O. O. Gruenz, Ir. and L. O. Schott in an article publishedin the Journal of Acoustical Society of America for September l949(volume 29) page 487. The principal feature ofV this generator is adetector followed in tandem by a shapnig network which accentuates theamplitudes of low frequency components at the expense of higher harmoniccomponent amplitudes. Preferably, each of these steps is carried out twoor more times in succession, and all of themV may be preceded by anauxiliary shaping step as by the first unit shown. With thisarrangement, as is more fully described in the patent and publicationreferred to above, the output of the generator 6 comprises single sharpspike of current which occurs at the instant of each major peak of thespeech signal wave, o-r between the principal zero and the major peak.For present purposes, the principal zero is the last zero value or axialcrossing of the speech wave preceding each of its major peaks.

When the speech is voiced, its waveform is periodic, and this markerpulse thus indicates the instant of inception of cach full period of thespeech wave, and the frequency at which such pulses are repeated is thefundamental pitch frequency of the speech wave.

For voiced sounds of speech, the train of marker pulses is appliedthrough the front fixed contacts of a relay 7 to a frequency divider 5,for example a multivibrator, proportioned to deliver one output pulsefor every N input pulse. The input terminal of the divider S, bearing atrain of marker pulses which recur at the recurrence rate of the majorpeaks of the speech wave, is connected to one input terminal 12 of abistable multivibrator il) while the output terminal of the divider 8,bearing a train of pulses whose repetition rate is l/N times thefundamental frequency of the speech wave, is applied to a second inputterminal 11 of the bistable multivibrator it). This multivibrator itsand the arrangement of its input terminals and its output terminals withrespect to its internal structure are such that application of a pulseto its inpult terminal 1l produces a signal on this output terminals,While application of a pulse to its input terminal 12 terminates thissignal. As a result, the output signal of the bistable multivibrator l@operates to close the switch at the inception of a period of the speechwave while cessation of this output signal, which takes place at theconclusion of this speech wave period, opens the switch 5 and soprevents conduction of the ensuing speech wave periods until the nextoutput pulse of the divider S appears. Thus, of every N successiveperiod of the speech wave, N-l

. of them are blocked.

' The switch 5 thus operates to chop any voiced speech wavesynchronously with its successive fundamental periods, passing one outof every N such period and leaving blank intervals in the signal on theconductor 13 corresponding to the remaining N -l of such periods. Thesignal as thus chopped may be transmitted to a receiver station withoutfurther change. it is advantageous, however, to turn the blank intervalsthus produced to account, either by reducing its vulnerability to noise,by reducing the bandwidth required to transmit it, or in some otherfashion. By way of example a time scale stretcher 14 is included intandem with the outgoing conductor 13. Its construction may be of anydesired sort, for example as described in the aforementioned patent ofH. W. Dudley. lt is controlled in its stretching action by the output ofthe marker pulse generator 6. If preferred, various electroniccounterparts may be employed instead of Dudley-s apparatus. Whatever thestructural details of the time scale stretcher 14 may be,

4 its operation is to stretch each full period which passes through thegate 5 over the ensuing N-l blank intervals.

The second control path 4 extends from the output terminal of themicrophone 1 to a discriminator 16 as between voiced sounds and unvoicedsounds. This unit is well known and may have any desired construction.For example, it may comprise a band-pass filter, a rectifier and alowjpass filter connected in tandem in the order named. The band-passfilter being proportioned to pass only the principal frequencycomponents of voiced sounds, e. g., those in the range 604,000 cyclesper second, and the low-pass filter being proportioned to pass onlyfrequencies of syllabic rates, e. g., those in the range 0-25 cycles persecond, the unit gives an output f of substantial magnitude when thesound waves impinging on the microphone 1 are of the voiced variety, andgives no such output signal when the sound is unvoiced.

The output signal of this discriminator 16 is applied to the winding ofthe relay 7 and serves to actuate it and to hold its moving contactagainst its front fixed contacts when the sound is voiced. This permitsthe output pulses of the marker pulse generator 6 to follow the pathhereinabove described. When, to the contrary, the sound is unvoiced themoving contact of the relay 7 falls against the back xed contact. lnthis condition of the relay 7 the output pulse train of the marker pulsegenerator is converted to a slowly varying signal by a low-pass filter17 and this signal is applied to a relaxation oscillator 1S whichdelivers pulses at a slowly varying but nearly regular rate through theback iixedcontact to the moving Contact of the relay 7. pulses of therelaxation oscillator 18 thereupon follow the paths hereinabovedescribed for the output pulses of the marker pulse generator 6.

The marker pulse generator 6 delivers a pulse for every major peak ofthe speech wave. When the speech wave is of the voiced character thesemarker pulses recur periodically at the fundamental pitch rate. When, tothe contrary, the sound is unvoiced, the marker pulses recur at randomlyspaced instants on the time scale, and at an average rate lying withinthe normal pitch range. For effective chopping of the wave of anunvoiced sound the chopping should occur in a regular fashion. It is thefunction of the combination of the relaxation oscillator 18 and thelow-pass lter 17 to substitute, at the control terminals of the gate 5and of the time scale stretcher, 14, a comparatively regular train lofpulses which recur at the average recurrence rate of the randomlydistributed pulses of the marker pulse train itself.

lf preferred, the apparatus shown may be modified in such a fashion thatthe relaxation oscillator 18 delivers a train of pulses which recur, notat the average repetition rate of the marker pulses during an unvoicedsound, but rather at a rate corresponding to the fundamental frequency-of the most recent voiced sound; i. e., `at the rate which obtained atthe instant of the change of the relay 7 from its voiced position to itsunvoiced position. Apparatus of this character is disclosed in ranapplication of H. L. Barney, Serial No. 459,333, filed September 29,1954, now matured into Patent 2,819,341, granted January 7, 1958.

The resulting signal, stretched in time or otherwise modified asdesired, may now be transmitted to a receiver station over a channel 19which is of much lower quality than would be required for transmissionof the unmodiiied speech wave. Thus, if stretching in time be employed,the bandwidth of the channel 19 is reduced by a factor N as comparedwith the bandwidth of a channel required to transmit the original speechwave.

Upon arrival at the input terminal 21 the receiver station the receivedsignal is first restored to its original dimensions in the time scale asby a restorer 22 which again may be as described in the aforementionedDudley patent or an electronic counterpart or refinement thereof.V

The output Inasr'nuch as any such apparatus must operate in synehronismwith the received wave, a period marker pulse generator 26 is shownconnected to the input terminal 21 and delivering its output periodmarker pulses to the control terminal of the restorer 22. Itsconstruction may be identical with that of the period marker pulsegenerator 6 at the transmitter station. In addition, the train of periodmarker pulses, one of which occurs for each period acually received, isconverted into a slowly varying voltage as by a low-pass filter 23. Asin the case of the transmitter, these marker pulses are always of thesame polarity so that no rectifier is required, and the low-pass filter23 operates to convert the pulse train into a voltage of varyingmagnitude proportional to the fundamental pitch of the received signaland of unvarying polarity.

After completion of the time scale restoration, the signal is applied toan interpolator which may comprise a plurality of delay devices 27connected in tandem. The first of these delay devices 27a is providedwith a number of lateral taps 28a, any one of which may be engaged by aconductive wiper arm 29a, and the other delay devices 27b, 27C, 27d aresimilar in every respect. The several wiper arms 29 are mounted on acommon shaft 30 arranged to be driven by a direct-current motor 31,preferably through a reduction gear box 32. The full length of eachdelay device 27 is such that the propagation time of a Wave through itfrom its input terminal to its most distant tap is equal to the longestfundamenal period to be expected; i. e., to the reciprocal of the pitchof the deepest bass voice.

As the speech directed into the microphone 1 progresses, its fundamentalfrequency changes and the output voltage from the low-pass filter 23changes proportionally. This voltage is added to a fraction, determinedby the position of a wiper arm 33 of the voltage which appears across aresistor 34 which shunts a battery 35. The sum of these voltages isconverted by an amplifier 36 to a driving current for the motor 31,which drives the wiper arm 33 through shaft 37. In accordance with thewell known behavior of servo systems, the motor 31 then rotates, drivingthe shaft 37 and the wiper arm 33 in a direction and through an anglesuch as to bring the net voltage input to the amplifier 36 to the valuezero. In doing so, it drives each of the several wiper arms 29 tosimilarly numbered taps on the several delay lines 27, and the tap atwhich each wiper arm comes to rest is that at which the propagation timefrom the input terminal of any delay line to that tap is equal to theinstantaneous value of the fundamental period of the speech wave.

As remarked above the output signal of the low-pass filter 23 is,proportional to pitch in contrast to period. Hence the wiper arms 29 ofthe delay devices 27 are located, in response to the movement of theshaft of the motor 31, at positions proportional to pitch. Because eachwiper arm is required to engage a particular tap of its delay line in afashion to match individual periods of the wave, an inversion operatonmust somehow be effected as between the controlling signal and resultingtap locations along the delay line. This is conveniently achieved simplyby numbering the several taps of each delay line starting with the tailend of that line, instead of with its input end, and by an appropriatenonuniform spacing among the taps in accordance with the requiredreciprocal relation.

The input terminal of the first delay device 27a is connected to oneinput terminal of a weighting network 40 and each of the several wiperarms 29 is connected to another such input terminal. In the exampleshown in which the value of the chopping factor N is 3, four delay lines27 are employed and the weighting network 40 has five input points. Moregenerally, for any value of the chopping factor N the number of delaylines 27 required is' 2N-2 and the number of input points to theweighting network 40 is 2N -l. Again in the example shown the Weightingfactors of the several input points to the weight- N-1 PFT Page.

A weighting network 40 of any desired` sort may be employed, aconvenient one being shown in Fig. 2 for the illustrative casein whichN=3. It operates in a manner well known in the art to give the properweights to the signals applied to its various input points and then toadd these weighted signals together in the fashion shown in the figure.

The output of thisnetwork 40 thus constitutes a correctly weighted sumof the various inputs. It is applied through an amplier 41 to areproducer 42 which delivers acoustic waves of artificial speech.

For a full understanding of the operation of the network 40, consider aninstant at which one of the received wave periods of the speech haspassed through the first and the second delay devices 27a, 27b and hasreached the third input terminal of the weighting network 40. It is thenreproduced without change, inasmuch as the weighting coefficient forthis input point is unity. One fundamental` period later the same wavehas passed through the third delay device 27c and appears at the fourthinput terminal of the weighting network 40. There it is multiplied bythe factor 273. At the same' instant the next transmitted wave periodarrives at the input point to the first delay device 27a and is suppliedby way of a conductor 43 tothe first input point of the weightingnetwork 40 Where it is multiplied by a factor 1/3. Hence, the reproducedwave is a mixture of 7a of the earlier wave and 1/3 of the later one. Itis reproduced as audible sound by the reproducer 42 in the first blankinterval following the earlier transmitted wave. Still later by onefundamental period of the speech wave, the earlier transmitted wave haspassed through the fourth delay device 27d and it is applied to thefifth input point of the weighting network 40 where it is multiplied bya factor 1A. At the same instant the following transmitted wave haspassedthrough the first delay device 27a and has arrived at the secondinput point of the weighting network 40 where it is multiplied by thefactor 2A. Thus, the reproduced wave is a mixture of Va of the earliertransmitted wave and 2/3 of the later one, and it occurs at a time suchthat it is inserted in the second blank interval. The next event whichoccurs is that the following transmitted wave has passed through thefirst two delay devices 27a, 27b, and is applied to the third inputpoint of the weighting network 40. From here on the cycle of operationsis repeated.

While the apparatus of Fig. l illustrates the principles of theinvention the shafts 30, 37, the mechanical wipers 29, 33, the follow-upmotor 31 and associated apparatus impose limitations on the speed withwhich it can operate. For these reasons an electronic counterpart of theapparatus of Fig. l may well be preferred. Such electronic apparatus, anexample of which is shown in Figs. 3 and 4,

'i is the same as the apparatus of Fig. 1, except ,QI the substitutionof a relay tree and its contacts for the wipers of Fig'. 1 and circuitsto control the windings of the relay tree .for the servo system of Fig.l. In order always to select a tap that will match the period of thedelay lines 27 with the pitch period, the pitch voltage is converted toa number representing the particular tap on each of the several delaylines which introduces a delay approximately equal to the pitch period.To this end the pitch voltage, derived by the filter 23 from the trainof marker pulses from the generator 26, is applied to a converter 50,whose details are shown in Fig. 4. The pitch voltage is sampled at theinstant of a period marker pulse and applied as the initial chargeacross a Miller type capacitor in a phantastron delay generator 51, `asdescribed, for example, by Chance,Hughes, et al., in Waveforms(Radiation Laboratories Series, volume 19, McGraw-Hill 1949), page 197.The phantastron 51 is of the monostable variety and its action isinitiated by each marker pulse. The output derived fromV the screen gridof the phantastron 51, is a rectangular pulse starting at the instant ofa marker pulse, and having a duration directly proportional to the pitchvoltage. The duration of the rectangular pulse developed at the screenmay be converted into a binary number by applying the pulse tothecontrol terminal of switch` or gate 52 which, when its conduction pathis thus established, admits a sequence of very rapid pulses from agenerator 53 into a string of scale-oftwo counters 54. In a typicalsystem the pulses are generated at a 100 kilocycle rate, so that thecounting process is completed in a small portion of a pitch period. Thebinary state of the group of scale-of-two counters, returned to zeroafter the completion of each count by `a reset circuit 55 under controlof each marker pulse, is now directly proportional to that tap of thedelay devices 27 whoseV numerical designation is most nearlyproportional to the fundamental period of the speech wave. The coils 56of a relay tree S7 are connected to the scaleof-two counters 54 in afashion such that the presence of a' l closes the relay contacts and thepresence of a 0 leaves them open. Thus the relay tree contacts operateto connect the proper tap of each delay device 27, as indicated by thecounters 54, to the input terminal of the next delay device 27. As arefinement, ,the delay devices 27` may be divided into two groups, andthe delays for similarly numbered taps in the two groups may be set attwo slightly different times; e. g., at times which differ by one pitchperiod, to avoid switching when signals are present in the delaydevices. A second counter and a delayed marker pulse generator may beemployed for this purpose.

Returning to Fig. 1, it was remarked above that the time scale stretcher14 at the transmitter station and the time scale restorer 22 at thereceiver station might take the form shown in the aforementioned H. W.Dudley patent. However, especially for cases in which the choppingfactor N has the value 3 or more, an electronic apparatus for carryingout these operations may be preferred. Fig. 5 shows one such electronicstretcher which employs a double beam storage tube 6i), for example atube of the type dsecribed in the RCA Review for March 1949 (volume l)page 59 and is common known as a Graphecho-n. This tube generatesseparate writing and reading electron beams 61, 62 so that onepitchperiod of the speech signal can be written on the mosaic of its targetanode 63 by the writing beam 61 and the reading beam 62 can scan thetarget 63 independently. Fig. also shows, in block diagram form,apparatus which acts to chop out an integral number of pitch periodsfrom the speech wave and stretch each retain'ed period to till the gapleft by the chopping, while Fig. 6 shows the waveforms which appear atthe various similarly labelled points of the circuit of Fig. 5. Thespeech wave (A) from the microphone 1 Yis applied to the marker Pulsegenerator 6 which produces e pulse J(1C) et the Ainitial .instant ofeach pitch period. The resulting period marker pulses arecon-r verted bythe low-pass filter 17 into a slowly-varying pitch voltage (D) which isdirectly proportional to the repetition rate of the period pulses (C).This pitch voltage (D) determines the rate of change of the sawtoothoutput wave (E) of a writing sweep generator 64 whose sweeps areinitiated by the period pulses (C). The result is a sawtooth wave thathas a constant amplitude, starts with a period pulse, and has the sameperiod as the fundamental voice period. This is the signal thatdetermines the writing rate of the beam 61 of the tube 60. A divider 65reduces the recurrence rate of the period pulse train (C) by a factor N,so that at its output (F) a pulse appears for every N pitch pulses (C).

Each output pulse (F) of the divider 65 operates lthe multivibrator 10and so opens the gate 5 to admit the resulting chopped speech wave (B)to the grid of the tube 60, thus to modulate the writing beam 61. At thesame time it initiates the sweep (G) of a readin'g sweep generator 66which may be a duplicate of the writing sweep generator 64, except forits sweep speed. As in the case of Fig. 1, the gate 5 is opened by eachof the pulses (F) for exactly one period of each N periods and is thenclosed by the next one of the period pulses (C). A low-pass lter 67controls the rate of decay of the reading sweep wave (G). The variouselectrodes and the erasing mechanism of the storage tube 60 are to beconnected to suitablevoltages and circuits to insure proper operation.The curve (H) shows the retained portions of the speech wave, stretchedin time, as they appear at the output terminal of the mosaic targetanode 6,3 rof the tube 60. Associated apparatus for chopping anaperiodic speech wave, not shown in Fig. 5, may bey identical with thatof Fig. l.

The operation of restoring the original time scale to the chopped andstretched speech can likewise be accomplished with an electronic storagetube 7,0 and associated circuits shown in Fig. 7 that are very similarto those described above in connection with the electronic stretcher ofFig. 5. This system employs the writing beam 71 of the storage tube itlto record the signal incoming at the terminal 21 as a charge pattern onthe mosaic of a target 73 at a rate that isV so adjusted that one periodof the input signal extends across the target. At the instant that thewriting beam 71 starts to write each period, Athe reading beam 72 startsto read the previous period at N times the writing rate. Although thesetwo electronA and recorded, there is a delay of one stretched period ibetween the reception of a stretched period and its reproduction with arestored time scale.

The controls for this tube operate as follows. Rcferrin'g to thewaveform diagram, Fig. 8, the curve (I), shows the stretched wave asreceived. the curve (H) of Fig. 6. The marker pulse generator 26produces a pulse (K) at the inception of each Astretched pitch period.These pulses initiate the sweeps of a Vwriting sweep generator 74 and areading sweep generator 75. A low-pass iilter 23 converts the train ofmarker pulses (K) into a voltage (L) that is proportional to theirrepetition rate; i. e., to the pitch of the original speech signal. Thispitch voltage (L) from the low-pass filter 23 controls the run down'rate of the writing sweep generator 74 so that it produces a sawtoothwave (M) that hasV a constant amplitude and is initiated by the pitchpulses (K). The reading sweep of the generator 75 is likewise initiatedby the pitch pulses (K), and the sweep rate is .controlled by the Output(L) of the `10W-pass .filter It is the same as i 23. The reading sweepgenerator 75 produces a sawtooth wave (P) that has N times the slope ofthe writing sweep generator wave (M). The result is a target current (Q)derived from the discharge of the mosaic of the target 73 of the tube 70which reproduces the input signal with a new time scale that is N timesthe original one.

The reading sweep generator 75 and the writing sweep generator 74 may beconventional, as shown in Fig. 9. A capacitor 77 is charged through acathode follower 73 to a preassigned voltage by each marker pulse (C, For K) and it is allowed to discharge through the high variationalimpedance of a pentode 79 that is con'nected as a constant current load.The magnitude of the equivalent constant current Ais approximatelyproportional to the pitch voltage, (D or L) applied to the control gridof the pentode 79. In this mann'er the capacitor 77 dischargesuniformly, and the rate of change of its voltage during discharge isproportional to the pitch of the speech wave. generator and the writingsweep generator is that the `capacitor in' the writing sweep generator74 is N times as If desired, the above system can be modified to reducethe delay by one period in the unstretched speech by initiating the sweep of the reading' beam 71 when the writing beam '72 has completed afraction of its excursion across the target 73. In this manner thereading beam 72 completes its reading operation of each period at thesame instant that the writing beam 71 cornpletes writing that sameperiod. This can be accomplished by initiating the-reading sweep from anamplitude discriminator that indicates when the writing sweep hascompleted the fraction of its excursion.

As indicated above, a speech wave is almost periodic in character, whichmeans that each of its periods is almost, but not quite, the same inform as the prior period. The departure from exact similarity representsa gradual change in any or all the features of the wave, namely, thephase distribution as between its cornponents, the duration of theperiod on the time scale, and its amplitude or power. The apparatusdescribed Vabove constructs artificial waves each of which is theproperly weighted sum of an earlier wave period and a later one. Hence,it `takes all of these gradually changing features into account.

Experiments have shown that the amplitude or power of the speech wave isits property which changes most rapidly and which, therefore, is chieflyresponsible in the change in the wave from each period to the next. Inaccordance with the invention in another of its aspects this one sourceof change is utilized to control reconstruction of artificial speech,the others being disregarded. This embodiment, which makes forsimplification of the apparatus, may be instrumented as shown in Fig.l0,

where a speech wave originating, for example, in a microphone 1 ispassed through a variolosser 80, preceded and followed by amplifiers 81,82. The output of the second amplifier 82 is passed through a rectifier83 and a low-pass filter 84 to provide a slowly varying control currentfor application to the'variolosser to adjust the magnitude of theimpedance which it interposes inthe voice path. These elementsconstitute a conventional volume compressor, and the control currentderived from the output terminal of therlow-pass filter 84 may also sThe only difference between the reading sweep 10 serve, in the fashionto be described, as a measurepf the instantaneous amplitude or power ofthe original speech. To this end it is transmitted over an auxiliarychannel 85,

The compressed output of the amplifier 82 is then chopped in the fashiondescribed in connection with Fig. l, and through the action of likeapparatus, with a chopping factor N as determined `by the constructionof the divider 8. The chopped speech may now be stretched in time as bya stretcher 14 orotherwise modified to take advantage of the choppingaction. It is next transmitted over a medium 19 to a receiver station.There itis first restored to its original dimensions on the time `scalethrough the agency of a timescale restorer 2,2 controlled by a markerpulse generator 26 'as described in -connection with Fig. l. At theVoutput point ofthe timescale restorer 22 one period out of every Nperiods appears, and it is the function of the apparatus to be describedto insert, in the remaining N -1 blank intervals, repetitions of theprior received period modified .only as tojtheir amplitudes. Thesuccessive repetitions may be .constructed much Vas described inconnection with Fig. `l by applying the chopped speech output of `thetime scale restorer 22 to a sequence of N -1 delay devices. Thus, in theillustration in which N :3, two such delay devices, 86a, 86h are shown.The delay which each one of `them introduces is equal to a singlefundamental period of the speech wave, and these delays `must thereforebe continuously varied as the period or pitch of the speech varies. Tothis end, they are shown as under the control of the output of themarker pulse generator 26, smoothed to a slowly varying pitch voltage bythe filter 23. `,The actual apparatus by which this pitch voltage`adjustsfthe delays in proportion to the speech wave period maybesimilar to `that shown in Fig. l. t

The output of the time scale restorer Vv22 is applied through an addingnetwork 86 to an amplifier 8,7 (a) without delay, (b) after a delay ofone fundamental period and (c) after a delay of two fundamental periods.The output of this amplifier 1,87 vthus includes, following each waveactually received, two successive repetitions of it. These waves are nowapplied to a volume expandor 88 which operates `to expand the `volumerange of the signal applied to it to the same extent as that by whichthe volume range of the original speech was compressed by thevariolosser 80, at the transmitter station. One convenient way ofachieving this result is to provide La negative feedback amplifier 39having in its feedback path a variolosser 90 identical with thevariolosser in the forward path of the compressor at the transmitterstation. The impedance interposed by this variolosser90 `is adjusted by,and is under `the continuous control of, `the amplitude control signaltransmitted by way `of the auxliary channel 85.

The output signal of the amplifier 89 may now be applied by way of asuitable coupling transformer to a sound reproducer 42.

Itis `a feature of the apparatusshown in Fig. lO, and especially of theemployment of a compressor-expandor combination therein, that theauxiliary amplitude control signal may be continuously applied to theentire output of the amplifier 89 including receivedl periods as well asrepeated periods. If preferred, and at the cost of some furthercomplexity of the apparatus, the compressor 80 and the expander 38 maybe dispensed with and each received period may be reproduced withoutchange, the auxiliary volume control signalibeing applied exclusively tothe N-l artificial repretitions of each received period.

The first form of the invention shown in Fig. 1, with or without therefinements thereof shown in Figs. 3 to 9, inclusive, comprises a linearor first order interpolator. The interpolation principle embodied inthese: figures may evidently be extended in two' or more different ways:

First, it may be extended to include interpolation of the second or ahigher order simply by appropriate modifications and extensions of theweighting network 4). Second, it may be extended to includeinterpolation based, not merely on the immediately preceding andfollowing periods, but on still earlier and later periods as well. Thisextension, which provides higher precision of the reconstructed wave,may be effected by appropriate modi cations and extensions of the delayand control circuits, as well as the weighting network.

Furthermore, the two principal forms of the invention shown in Figs. land l0, respectively, are not to be taken as mutually exclusive.Situations may arise in which it is of advantage to supplement theactio-n of an interpolator, for example, a first order interpolator asshown in Fig. l, with an auxiliary amplitude control signal as shown inFig. 10. Such a combination is within the contemplation of theinvention.

What is claimed is:

l. Speech transmission apparatus which comprises a source of a speechwave consisting of a sequence of wave portions, means for eliminatingcertain individual ones of said portions to leave blank intervals, meansfor transmitting to a receiver station wave portions preceding andfollowing each blank interval, means for developing, at said receiverstation, a sequence of artificial wave portions of which the amplitudesfollow the amplitude variations of the original speech waves, and meansfor intercalating at least one of said artificial wave portions in eachblank interval, thereby to reconstruct said original wave.

2. Speech transmission apparatus which comprises a source of a speechwave consisting of a sequence of wave periods, means for eliminatingcertain individual ones of said periods to leave blank intervals, meansfor transmitting to a receiver station wave periods preceding andfollowing each blank interval, means for developing, at said receiverstation, a sequence of artificial wave periods, means for causing theamplitudes of said artificial wave periods to follow the amplitudevariations of the original speech wave, and means for intercalating saidsequence in each blank interval, thereby to reconstruct said originalwave.

3. Speech transmission apparatus which comprises a source of a speechwave consisting of a sequence of wave periods, means for eliminatingcertain individual ones of said periods to leave blank intervals, meansfor transmitting to a receiver station wave periods preceding andfollowing each blank interval, means for developing, at said receiverstation, artificial, wave periods each of which is a weighted sum of apreceding and a following trans mitted wave period, and means forintercalating at least one wave of said sequence in each blank interval,thereby to reconstruct said original wave.

4. Speech transmission apparatus which comprises a source of a speechwave consisting of a sequence of wave periods, means for eliminating N-lof each group of N successive ones of said periods to leave blankintervals, means for transmitting to a receiver station wave periodspreceding and following each blank interval, means for developing, atsaid receiver station, a sequence of N-l artificial wave periods Pa, Pb,Pc PN 1, means for proportioning said artificial wave periods inaccordance with the formulae:

wherein P1 represents the period immediately preceding a blank intervaland P2 represents the period immediately following said blank interval,and means for intercalating said artificial wave periods sequentially ineach blank interval, thereby to reconstruct said original wave.

5. Speech transmission apparatus which comprises a source of a speechwave consisting of a sequence of wave periods, means for developing anauxiliary signal proportional to the amplitude of said speech wave,means for eliminating certain individual ones of said periods to leaveblank intervals, means for transmitting to a receiver station waveperiods preceding and following each blank interval, means for alsotransmitting said auxiliary signal, means for developing, at saidreceiver station, a sequence of artificial wave periods, each of whichis a substantial repetition of the preceding transmitted wave period,means for varying the amplitudes of said repetitions under control ofsaid auxiliary signal, and means for intercalating at least one of saidrepetitions in each blank interval, thereby to reconstruct said originalwave.

6. Speech transmission apparatus which comprises a source of a speechwave comprising a sequence of wave periods, means for eliminating fromsaid sequence N-l of each group of N successive ones of said waveperiods to leave blank intervals, means for transmitting to a receiverstation wave periods preceding and following each blank interval, means,at said receiver station, for developing for each blank interval andfrom the transmitted waves which precede and follow said blank intervala group of N-l artificial wave periods, means for proportioning saidseveral artificial wave periods as variously weighted sums of saidpreceding and following wave periods, and means for intercalating saidartificial wave periods in said blank interval.

7. Speech transmission apparatus which comprises a source of a speechwave comprising a sequence of wave periods, means for eliminating fromsaid sequence N -l of each group of N successive ones of said waveperiods to leave blank intervals, means for transmitting to a receiverstation wave periods preceding and following each blank interval, means,at said receiver station7 for mixing the transmitted waves which precedeand follow each of said blank intervals in various proportions, to forma group of N *l artificial wave periods, means for arranging saidartificial wave periods in order of increasing emphasis of the followingwave and decreasing emphasis of the preceding wave, and means forintercalating said ordered artificial wave periods in said blankinterval.

8. Speech transmission apparatus which comprises a source of a speechwave comprising a sequence of wave periods, means for eliminating fromsaid sequence N -l of each group of N successive ones of said waveperiods to leave blank intervals, means for transmitting to a receiverstation wave periods preceding and following each blank interval, means,at said receiver station, for developing for each blank interval andfrom the transmitted wave which precede and follow said blank interval agroup of N l artificial wave periods, means for proportioning saidseveral artificial wave periods as variously weighted sums of saidpreceding and following wave periods, means for variously delaying eachof said artificial wave periods by an integral number of fundamentalpitch periods of said speech wave to form a sequence of artificial waveperiods, the members of which sequence differ progressively from end toend of said sequence, and means for intercalating said sequence in saidblank interval.

9. Apparatus as defined in claim 8 wherein said delayed means comprises2N-2 delay devices, coupled together in tandem, means for applying eachreceived wave to the input terminal of the first of said devices, meansfor deriving a delayed wave from the output terminal of each of saiddevices, and means for additively combining said delayed waves.

10. In combination with apparatus as defined in claim 9, means forderiving a period control signal from received Waves, and means foradjusting the propagation length of each of said delay devices undercontrol of said period control signal to introduce a delay equal to thefundamental period of said speech Wave.

11. Speech transmission apparatus Which comprises a source of a speechwave comprising a sequence of wave periods, means for eliminating fromsaid sequence N -1 of each group of N successive ones of said Waveperiods to leave blank intervals, means for developing an auxiliarysignal representative of the changing character of said speech Wave,means for transmitting to a receiver station wave periods preceding andfollowing each blank interval, means for also transmitting saidauxiliary signal to said receiver station, Ameans, at said receiverstation, for developing for each blank interval and from the transmittedwave which precedes said blank interval a group of N -l artificialrepetitions of said preceding transmitted wave, means for variouslydelaying each of said repetitions by an integral number of fundamentalpitch periods of said speech wave to arrange said repetitions in anordered sequence, means for varying the amplitudes of said repetitionsunder control of said auxiliary signal, whereby the members of saidsequence difer progressively from end to end of said sequence, and meansfor intercalating said sequence in said blank interval.

12. Apparatus as dened in claim 11 wherein said delaying means comprisesN-1 delay devices, coupled together in tandem, means for applying eachreceived wave to the input terminal of the iirst of said devices, meansfor deriving a delayed wave from the output terminal of each of saiddevices, and means for additively combining said delayed waves.

13. In combination with apparatus as defined in claim 12, means forderiving a period control signal from received waves and means foradjusting the propagation length of each of said delay devices undercontrol of said period control signal to introduce a delay equal to thefundamental period of said speech Wave.

14. Speech transmission apparatus which comprises a source of a speechwave comprising a sequence of wave periods, a volume-range compressorfor said waves, means for deriving from said compressor an auxiliarysignal representative of the amplitude variations of said speech wave,means for eliminating from said sequence N-1 of each group of Nsuccessive ones of said wave periods to leave blank intervals, means fortransmitting to a receiver station Wave periods preceding and followingeach blank interval, means, at said receiver station, for developing foreach blank interval and from the transmitted Wave which precedes saidblank interval, a group of N -1 repetitions of said preceding wave,means for variously delaying said repetitions by an integral number offundamental pitch periods of said speech wave to form a sequence ofartificial Wave periods, means for intercalating said sequence in saidblank interval to form a reconstructed Wave of compressed volume rangewithout blank intervals, and means for varying the.` amplitude of saidreconstructed wave under control of said auxiliary signal.

References Cited in the tile of this patent UNITED STATES PATENTS2,098,956 Dudley Nov. 16, 1937 2,732,424 Oliver Jan. 24, 1956 2,766,325DiToro Oct. 9, 1956 UNITED STATES PATENT OFFICE Certificate ofCorrection Patent No. 2,860,187 November 11, 1958 Edward E. David, Jr.,et al.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction and that the saidLetters Patent should readA f as corrected below.

Column l1, lines 68 and 69, the formulaJ should appear as shown belowinstead of Signed and. sealed this 17th day of March 1959.

[SEAL] Attest: KARL H. AXLINE, Attestz'ng 0770e1'.

ROBERT C. WATSON, ovmnssoner of Patents.

